Add apex_set module.

apex_set takes an .apks file that contains a set of prebuilt apexes with
different configurations. It uses extract_apks to select and install the
best matching one for the current target.

Bug: 153456259
Test: apex_test.go
Test: com.android.media.apks
Change-Id: I1da8bbcf1611b7c580a0cb225856cbd7029cc0a7
diff --git a/apex/apex.go b/apex/apex.go
index 9525ff2..0b68ae4 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -722,6 +722,7 @@
 	android.RegisterModuleType("apex_defaults", defaultsFactory)
 	android.RegisterModuleType("prebuilt_apex", PrebuiltFactory)
 	android.RegisterModuleType("override_apex", overrideApexFactory)
+	android.RegisterModuleType("apex_set", apexSetFactory)
 
 	android.PreDepsMutators(RegisterPreDepsMutators)
 	android.PostDepsMutators(RegisterPostDepsMutators)